Before each roll you receive an encrypted SHA1 string that contains a Secret Key-RolledNumber-JackpotType, that means the roll number is already predefined before you even click roll dice so there is no way for anyone to cheat.
After each bet ended you can verify its integrity as the server will show you the string unencrypted so if you run a SHA1 on it you will see it was the exact string it was given at the beginning.
In theory, you can just always return "jackpot 0" and never let anyone win a jackpot. Also in theory, if a player has a specific play pattern (for example: under 2x), you could generate outcomes that make the player lose (and obviously in theory, if you would do that, a player could abuse that as well.)
Anyway, basically for a real provably fair mechanism, you should require input from the player like a clientseed (after the player gets the hashed serverseed already.) The end result and jackpot is then calculated with both seeds.